Transmission system



Oct. 30, 1945. H. s. BLACK TRANSMISSION SYSTEM Filed April 4, 1944 /NVEN To@ H S. BL A CK A r 'TOR/MEV Patented Oct. 3 0, 1945 TRANSMISSIONSYSTEM Harold S. Black, Elmhurst, N. Y., assigner to Bell TelephoneLaboratories, Incorporated, New York, N. Y., a corporation of New YorkApplication April 4, 1944, Serial No. 529,498

5 claims.

This invention relates to a system for the transmission of intelligenceand more particularly to a long multiplex carrier system.

An object of the invention is the lessening of impairment of signalstransmitted over a multiplex carrier system.

A more specific object ofthe invention is the lessening of impairment ofsignals transmitted over a. multiplex carrier system due tointerference'factors of a type that, are not variable with frequence,`i. e., that are non-frequency dependent.- i

`A troublesome factor in the operation of a multiplex carrier system isthe occurrence of various types of interference which are functions offrequency, i. e., which vary in intensity with changes in frequency.Among such types of interference are static and cross-talk. It isapparent that as signals are transmitted over Such a system in amultiplicity of respective frequency bands some signals will besubjected to interference factors of greater intensity than others.Certain circuits accordingly may be excessively noisy whereas othersmaybe unnecessarily quiet; it is the condition of the noisiest circuit thatis the limiting factor in the design of the system.

In order to reduce the effects of such frequencydependent types ofinterference factors, it has previously been proposed, as disclosed forexample in Patent 2,154,594 issued April 18, 1939,

to M. A.\Weaver, to systematically assign each signal to successivelydifferent frequency channels as it progresses through the system. Thesystem of successive assignments is such that the intensity ofcross-talk or other frequency-dependent interference factor issubstantially the same for all circuits and is substantially less thanit would be in the noisiest circuit if the signals were conned torespective frequency channels.

Interconnecting or mixing different transmission element sections in thegeneral manner discussed above iscommonly referred to as frogging andthe specific type disclosed in the Weaver patent as channel frogging.

In addition to the impairment by frequencydependent interferencefac'tors, signals transmitted overa multiplex carrier system may also besubjected to impairment by an additional factor of a type that. is notfrequency-dependent. The impairment referred to is that resulting fromthe effect upon the transmission regulators of certain high volumepulses originating in the speech channels for example such pulse's asresult from ringing.l

Itis customary, of course, in the operation 0f long multiplex carriersystems to provide means for automatically maintaining the speech vlevelsubstantially constant in spite of various unstable factors tending tocause changes in the level. Forv example, so-called flat gain regulatorsare ordinarily provided at intermediate` points in the system forcontrol of amplifier gain with respect to input variations of the typein which the change in gain is substantially uniform for all frequenciesover the transmitted range. These iiat gain regulators are designed tohold practically constant the output power of talking volumes over theworking range of input levels. However, the high volume pulses of thetype referred to above often considerably exceed the average power levelfor which the system is set up and with. regard to which the flat gainregulators are designed. The initial flat gain regulator aifected by thepulse is not able to fully compensate for it therefor, as it would ifthe change were within the working range of the system, so thatsubsequent regulators are affected in decreasing degrees until the pulseis finally fully compensated for.

The high volume pulses of the type referred` to are usually very shortin duration, of the order of one second, and the net effect onregulation where only a few repeaters are involved is: not severe. Inthe instance of a long system Where many repeaters are involved,however, the cumulative effect on many repeaters may result in seriousimpairment of transmission. Where many regulators in tandem are involvedthe result of the action of the pulse thereon is the production ,of atransient in the approximate form of a damped sine wave (decreasing inamplitude with each regulation). The frequency of this damped sine waveis a function of the number of repeaters and the over-all magnitudeofthe effect is a function of the length of the circuit. Such a transienteffect is harmful to transmission over the line affected, particularlywith respect to telegraph channels carrying telegraph signals which, inthe instance of a long multiplex carrier system are usually superimposedupon one or more speechchannels; the reduction in gain at the successiverepeaters of course, tends to upset the normal level of the speechtransmission. The telegraph, however, is more sensitive to these upsets.In fact, it has been discovered that spurious transmission variations ofthis nature which are insufficient to affect or seriously impair theperformance of message or high quality program circuits are,nevertheless, large enough in magnitude to seriously interfere with thecommercial transmission of ordinary telegraph messages.

In view of the fact that the disturbance of the type referred to is notfrequency-dependent and as the transient disturbance set up in onechannel of the system affects all other channels of the systemsimilarly, transposition of channels of a single system, as set forth inthe Weaver patent referred to above, is ineffective with respect to thistype of transmission impairment.

In accordance with a feature of the present invention frequency channelsof one system are interconnected to frequency channels of a differentsystem whereby to reduce the non-frequency-dependent transmissionimpairment.

In accordance with another feature of the invention the order of thefrequency channels is changed as they are interconnected whereby toreduce the effects of frequency-dependent impairment as well asnon-frequency-dependent impair-- multiplex carrier systems each of whichcom- Y prises a plurality of different frequency channels and includes aplurality of intermediate stations, frequency channels of one of saidsystems are systematically interconnected to frequency channels ofothers of said systems at the intermediate stations. Theseinterconnections between channels of different systems are made as partof the permanent installation. While this results in a long throughcircuit being affected more frequently by high volume pulses, such asthose caused by rings on other channels, the magnitude of thetransmission variation on any of the circuits is perceptibly less due tothe fact that the 1 pulses affect the channels over a shonter distance,

for example, between two intermediate stations only rather than over anentire system. An improvement in the over-all transmission results,therefore.

A full understanding of the arrangement contemplated by the presentinvention as well as appreciation of the several novel and valuablefeatures thereof may be gained from consideration of the followingdetailed description and the annexed drawing, in which the single figureis a schematic diagram of a group of three multiplex carriertransmission systems arranged in accordance with features contemplatedby the present invention.

Referring now to the drawing there is schematically illustrated a groupof three multiplex carrier systems numbered No. I, No. 2 and No. 3 eachof which includes three separate frequency channels and each of which islprovided with a west terminal, an east terminal and two intermediatestations. A plurality of repeaters or amplifiers are included in eachlink of each system, i. e., the line section connecting the terminalsand the intermediate stations. As indicated schematically each channelis brought down to voice i. e., the modulated carrier wave isdemodulated to give the voice frequency wave at the intermediatestation, before the channels are' frogged; after interconnection thechannels are again raised to carrier frequency by modulation fortransmission over the next subsequent link, It will be understood thatautomatic transmission regulators are provided at each repeater point orat other suitable locations in the system; these regulators may forexample, be of the type disclosed in Patent 2,179,915, issued November14,

1939, to R. R. Blair. It will be understood also that the group may,.andusually does, comprise more than three different systems and that? @26hsystem may include more than three different frequency channels.Further, in a long transmission system of the type in connection withwhich the present invention is particularly applicable, more than twointermediate stations may be provided. Specific application of theinvention to a group of 12 systems each of which comprises 12 separatefrequency channels will be outlined subsequently.

As illustrated, frequency channel No. I of system No. I is .transmittedover link No. I tothe first intermediate station. Here it is broughtdown to voice frequency and, in accordance with the features of thepresent invention, is then transferred, interconnected or frogged tofrequency channel No. `2 of system No. 2. After being restored tocarrier-frequency it passes over link No. 2 of system No. 2. Similarly,channel No. I of system No. 2 is transferred at the rst intermediatestation to frequency channel No. 2 of system No. 3. The manner ofinterconnecting the other frequency channels at the first intermediatestation as well as the interconnections at the second station will beapparent from the figure.

In order to clearly disclose fthe improvement in transmission whichresults from this novel plan of frogging systems, let it be supposedthat a high volume pulse is produced, for example, by

a ring in channel I of system I at the West terminal. Assuming, first,that the systems are not frogged or interconnected, i. e., that featuresof the present invention are not provided, the pulse would traverse bothlinks I and 2 of system I (as well as subsequent links, if any)affecting the regulators of both links. Channels I, 2 and 3 of systemNo. I would allv be affected in the same manner and the transitoryfluctuations in net loss resulting in system No. I would be in.accordance with the cumulative effect resulting from the dynamicresponse of the regulators of both links. Now let us suppose that thesystems are transposed or frogged in the manner contemplated by thepresent invention and as illustrated in the drawing. In this case theoccurrence of the high volume pulse in channel No. I

of system No. I will'aifecrt transmission over channels No. 2 and No. 3of system No. I only so far as the regulators of link No. I areconcerned and will have no effect on the regulators of system No. I overlink No. 2, i. e., the effect upon the over-all transmission over systemNo. I will be only a fraction of that prevailing without the arrangementof the present invention. Transfer of the pulse to channel No. 2 ofsystem No. 2 will result in affecting transmission over channels No. Iand No. 3 of system No. 2'but only with respect to the regulators oflink No. 2 and will have no effect on the regulators of system No. 2over link No. I.

While, as will .be apparent from the above, the arrangement of theinventioni'esults in each long distance circuit being` affected morefrequently by rings and the like than would be the case otherwise, thisis more than oifset by the improvement in over-alltransmissicn whichresults from the fact that in each instance the number of regulatorsaffected is materially lesscned.

It is contemplated by the invention that both frequency-dependentimpairment of transmission and non-frequency-dependent impairment oftransmission may be reduced by changing the order of the frequencychannels in the general manner taught by the Weaver patent referred toabove at the time the channels are interconnected among differentsystems in the manner described above. It is also contemplated thateither` system frogging alone or the combination of system frogging andfrequency frogging may be applied to groups of any number of carriersystems each of While certain specific embodiments of the inwhichsystems comprises any number of frequency channels.

Applicant has worked out as a general guide for combined system andfrequency frogging in accordance with his invention the formula thatchannel N of system M should be interconnected to channel N+1 on systemM+N Where N and M are any real numbers. Applying this to a group ofsystems in which channel N is No. 3 and system M is No. 4 then channelN, (3) of system M, (4), would be interconnected at the intermediatestation in question to channel No. 4, (N+1), of system No. 1, (M+N)Applying the above formula in the instance of a group of 12 carriersystems each of which systems comprises 12 different frequency channels,we may set up the following general guidance table:

Table A table showing the general manner in which the 144 circuits from12 terminals of 12 systems may be interconnected to the 144 voicefrequency circuits associated with the corresponding 12 terminals of the12 outgoing systems, each system having 12 separate frequency channels.The plan of interconnecting is the same at all frogging points. (M isthe number of the incoming system and N is the number of the channelthat is frogged to channel N+1 on outgoing system N+M.)

Channel System 1 2 s 4 12 N N+1 MN-i-M N-l-M N-x-M N+M N11-M 1 2 2 a 4 51 2 3 a 4 5 e 2 a 4 4 5 e 7 r s 4 5 5 6 7 s 4 5 t c 7 s 9 /5 e 7 7 s 91o e 7 s s 9 1o 11 1 7 s 9 9 1o 11 12 s 9 1o 1o 11 12 1 r 9 1o 11 11 121 `2 i 1o 11 12 12 1 2 s 1 11 12 1 1 2 s 4 12 As an example of theapplication of the above table, let us assume we wish to determine theproper frogging of channel 2 of system 3. In accordance with thegoverning formula of the table, channel 2 of system 3 should be froggedto channel 3 (N+1) of system 5 (N+M). Again let us consider the froggingof channel 8 of system 7; here We will go to channel 9 (N+1) of system 3(N+M=15 or 15-12--3).

The above plan of frogging or interconnecting may be applied to a groupof any number of systems each of which Systems is made up of anynumberof separate frequency channels. When applied, substantialimprovement in transmission is achieved both with respect tofrequencydependent impairment and non-frequency-dependent impairment.

detailed disclosure, the invention is not, of course, limited in itsapplication to such embodiments. The embodiments disclosed should belooked upon as illustrative of the invention and not as restrictivethereof.

What is claimed is:

1. In a group of multiplex carrier systems each of which systemscomprises a plurality of separate frequency channels and includes twoterminals and an intermediate station, the method of reducing bothfrequency-dependent transmission impairment and non-frequency-dependenttransmission impairment which comprises the steps of interconnectingfrequency channels of one of said systems t0 frequency channels ofanother of said systems at said intermediate station and changing theorder of the frequency channels as they are interconnected.

2. In a group of multiplex carrier systems wherein the number of systemsincluded is greater than M and wherein each system includes a number ofseparate frequency channels greater than N, M and N being any realnumbers, and wherein each system includes two terminals and anintermediate station, the method of reducing bothnon-frequency-dependent transmission impairment and frequency-dependenttransmission impairment which comprises the step of interconnecting atthe intermediate station frequency channels of one of said systems tofrequency channels of others of said systems in accordance with the planwhereby channel N of system M is interconnected to channel N+1 of systemM+N.

3. A transmission medium comprising a plurality of multiplex carriersystems e-ach of which comprises a plurality of separate frequencychannels, an east terminal, a west terminal and an intermediate stationfor each of said systems, and means at said intermediate station forpermanently interconnecting frequency channels of one of said systemseach to a frequency channel of a diierent other of said systems.

4. A transmission medium comprising a plurality of multiplex carriersystems each of which comprises a plurality of separate frequencychannels, an east terminal, a West terminal and an intermediate stationfor each of said systems and means at said intermediate station fordistributing frequency channels of one of said systems among frequencychannels of others of said systems and for changing the order of saidfrequency channels as they are distributed over said other systems.

5. A transmissionmedium comprising a greater number than M of multiplexcarrier systems, each of which systems comprises a greater number than Nof separate frequency channels, M and N being any real numbers, an eastterminal, a west terminal and an intermediate station for each of saidsystems, and means at said intermediate station for distributingfrequency channels of one of said systems among frequency channels ofothers of said systems in accordance with a plan of distribution wherebychannel N of system M is interconnected to channel N+1 of system M+N.

HAROLD S. BLACK.

